Nonlinear dynamics of free-carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using
intense few-cycle pulses. Techniques as Z-scan, THz-pump / THz-probe, and optical-pump/ THz-probe are employed to
explore nonlinear interactions in both n-doped and photoexcited systems. The physical mechanism that gives rise to such
interactions is found to be intervalley scattering.
Terahertz emission from n-type (100), (110) and (111) InAs crystals have been measured as a function of the sample orientation. Emission was excited using 120 fs Ti:Sapphire laser pulses at an incident angle of 45° with fluences of approximately 1-2mJ/cm2. The data is shown to match the behavior expected for optical rectification at the surface, with small contributions from bulk optical rectification and photo-carrier diffusion. Thus, at fluences employed in the present study, it appears that the dominant mechanism for generating THz radiation is optical rectification at the surface.